Lighting devices have been developed that make use of Light Emitting Diodes, LED's, for a variety of lighting applications. Owing to their long lifetime and high energy efficiency, LED lamps are nowadays also designed for replacing traditional fluorescent lamps, i.e. for retrofit applications. For such an application, a retrofit LED tube is typically adapted to fit into the socket of the respective lamp fixture to be retrofitted. Moreover, since the maintenance of a lamp is typically conducted by a user, the retrofit LED tube should ideally be readily operational with any type of suitable fixture without the need for re-wiring the fixture.
Such a retrofit LED tube is, for example, disclosed in US 2015/0198290. Here, an LED lamp arrangement is disclosed for replacing a fluorescent lamp in a luminaire having a ballast for supplying power to the lamp. The LED lamp arrangement comprises a plurality of LEDs arranged in a plurality of groups, wherein the groups of LEDs are connectable in a plurality of circuit configurations, including at least a first circuit configuration, and a second circuit configuration having a different circuit arrangement of the groups of LEDs in which at least a portion of the groups of LEDs are connected into the circuit differently than in the first circuit configuration.
One of the drawbacks of the retrofit LED tubes is related to step dimming. Step dimming is commonly applied in professional applications such as warehouses, where a multi lamp luminaire lighting system is switched between full light output in case of occupancy and background light level in case of no occupancy. In these applications, the lighting is often not switched off during operating hours due to safety reasons. In the off-hours, the power to the entire system is switched off from a central location with, for example, a cabinet-based sweep timer.
On many legacy ballasts fitted lamps it is not possible to dim a LED tube lamp below a certain amount of light level. As below a certain amount of light level the power factor correction of the ballast gets instable, as the ballasts are not designed to function in an open circuit, i.e. without a lamp connected to it. In addition it is difficult with these known ballasts to enter stand-by mode for all loads running in idle state. The reason is that the ballast has open and short circuit protective circuitry.